Switched-Mode LED Drivers vs. Linear LED Drivers

Abstract: LED drivers can be classified into linear converters as well as switch-mode converters, each with advantages and disadvantages.

In LED lighting system, driving circuits are extremely important and account for the performance and the cost of overall system, therefore, the design of driving circuit is of critical importance for the LED lighting system. The LED driving circuits can be grouped into linear converters and switch-mode converters, each with pros and cons.

A switching mode power supply (SMPS) rectifies AC power coming from an external power source to obtain DC power, converts the DC power into AC power by carrying out a switching operation, changes a voltage of the AC power utilizing a transformer, and rectifies and smoothes the transformed AC power, in so doing delivering smoothed DC power. A switched-mode power supply normally include a switching device that, when switching on and off, stores energy in an inductor or other energy storage element and discharges the stored energy to an output of the switched-mode power supply. The switching device may be manipulated by a control circuit or controller, which outputs switching signals to turn the switching device on and off. Switching power supplies contain a switching voltage source using, as feedback, the measured output current rather than the usual output voltage. The switching power supply and current feedback circuit are relatively cumbersome because of the use of current-carrying coils, high-frequency capacitors and flyback diodes. The SMPS produces the predetermined magnitude of DC power via a high-speed switching operation to cause much noise, thus ultimately causing interference, which adversely impacts the bordering circuit elements. With the intention to compensate for this adverse impact, the SMPS typically add a noise filter and the like must be additionally provided, thereby ending up with an increase in volume and weight and consequently raising the manufacturing cost. Furthermore, if the storage capacitor is an electrolytic capacitor, it will limit lifespan of the LED driver.

A linear driver generates a fixed current that may vary marginally with changes in voltage. Linear driver is considered one of the major driving schemes for LED lighting system and has benefits of simple design and immunity to electromagnetic interference (EMI). Owing to their small size and low-cost, simple linear driver circuits have created extensive interest in field of the LED drivers. Linear drivers can be directly plugged into utility alternating current (AC) voltage and power up an array of LED devices without resorting to bulky and costly switching-mode LED drivers. Nevertheless, a linear power supply may cause inefficient use of energy, as a result of heat dissipation in the power supply. Linear drivers are only advantageous when the load voltage is close to the input voltage. While switching drivers are employed when the input voltage differs (either higher or lower) than the output. That is, the linear driver ceases to work when the overall forward voltage of the LED is above the input voltage. Switched-mode power supplies may be constructed to operate much more efficiently than linear power supplies. Switch mode power supplies can effectively convert electrical power from the source to a load, or to a number of different loads, with each corresponding to a different output. If the driver is positioned near the LEDs, the additional heat can cause the LEDs to operate at a raised temperature, shortening their lifetimes. Also, flickering in LED lamps may appear due to the fact pulsating current will introduce light flicker that varies at twice the power line frequency.